Around 1885 or 1886 the term "No Man's Land" became widely applied to the Public Land Strip. True to the plain language of the old West, the nickname referred simply to the fact that no man could legally own land in the Strip.
another answer is:
No man's land is land that is unoccupied or is under dispute between parties who leave it unoccupied out of fear or uncertainty. The term was originally used to define a contested territory or a dumping ground for refuse between fiefdoms.